The process for making sintered metals essentially includes mixing of powder as a raw material, compaction, sintering and after-treatment (heat treatment). Although the sintered products can be produced only through these essential steps, in many cases, additional steps or various treatments are performed between or after the essential steps according to requirements.
For instance, Japanese Patent Application First Publication No. 1-123005 discloses a process comprising the steps of compacting a mixed powder to form a preform, provisionally sintering the preform to form a metallic powder-molded body, re-compacting (cold forging) the metallic powder-molded body and then sintering (substantial sintering) the recompacted body.
Specifically, in the conventional process, the re-compaction (cold forging) step of the metallic powder-molded body is constituted by a provisional compaction step and a substantial compaction step. The metallic powder-molded body is provisionally compacted after applying a liquid lubricant to a surf ace thereof, and exposed to negative pressure to absorb and remove the lubricant therefrom. Then, the metallic powder-molded body is subjected to substantial compaction step.
Since these steps allow the lubricant to still remain in an interior of the preform, micropores within the preform can be prevented from being collapsed and eliminated, thereby inhibiting the preform from suffering from a porous structure. As a result, the density of the obtained product increases up to 7.4-7.5 g/cm3, thereby enabling the product to exhibit a higher mechanical strength than those of the prior arts.
In the above conventional case, an attention has been mainly paid to the re-compaction step of the molded body, i.e., it has been intended to enhance the density thereof by the re-compaction step in order to obtain a product having a relatively high mechanical strength. However, the product obtained by the re-compaction step shows only a limited mechanical strength.
Consequently, in order to further enhance the mechanical strength of the product, it has been considered to be effective to increase a carbon content of the product, i.e, increase an amount of graphite added to a metal powder. However, in general, when the amount of graphite added increases, the molded body is deteriorated in elongation, and shows an increased hardness, thereby causing problems such as deteriorated deformability upon the re-compaction of the molded body and, therefore, difficulty in conducting the re-compaction step.
For example, in a pamphlet entitled “The Second Presentation of Developments in Powder Metallurgy,” published by Japan Powder Metallurgy Association (Nov. 15, 1985), page 90, it has been described that a metallic powder-molded body having a carbon content of 0.05 to 0.5% exhibits an elongation of 10% at most, and a hardness of HRB 83. However, it is known from experience that a metallic powder-molded body having an elongation of not more than 10% and a hardness of more than HRB 60 is difficult to be re-compacted. For this reason, it has been required to obtain a metallic powder-molded body having a still higher elongation, a low hardness and an excellent deformability.
The present inventors have continuously made intense studies for producing various structural machine parts having a high mechanical strength due to the use of sintered metals. As a result, it has been recognized that when machine parts are manufactured by provisionally sintering a preform to form a metallic powder-molded body, re-compacting the molded body and subjecting the re-compacted body to substantial sintering, the metallic powder-molded body bears important factors determinate to qualities of the obtained machine parts. Therefore, it is necessary to obtain a molded body having a predetermined graphite content, a large elongation, a low hardness and an excellent deformability. Based on the above recognition, the present inventors have conducted further researches.
As a result of the researches, it has been found that the properties of the metallic powdermolded body having a predetermined graphite content, especially elongation and hardness thereof which are important properties for facilitating the recompaction, are influenced and determined by a density of the preform prior to the formation of the molded body, a structure of the molded body obtained by provisionally sintering the preform, and the configuration of carbon contained in the molded body.